Global Navigation Satellite System (GNSS) receivers determine position by computing the relative time of arrival (TOA) of ranging signals that are simultaneously transmitted from a plurality of GNSS satellites orbiting the earth and/or pseudolites (collectively referred to herein as “GNSS sources”). GNSS sources transmit timing and satellite orbital (e.g. ephemeris) data in addition to ranging signals. As described herein, GNSS sources can include the United States Global Positioning System (GPS), the Russian Glonass system, the European Galileo system, any system that uses satellites from a combination of satellite systems, or any satellite system developed in the future (collectively called as “SPS” or “Satellite Positioning System”). Furthermore, some position determination systems utilize pseudolites or a combination of satellites and pseudolites. Pseudolites are ground-based transmitters that broadcast a ranging code, such as a PN code (similar to a GPS or CDMA cellular signal), modulated on a carrier signal which may be synchronized with time provided by an SPS. Pseudolites are useful in situations where SPS signals from an orbiting satellite might be unavailable. The GNSS sources, as referred herein, include SPS, pseudolites or a combination thereof.
GNSS receivers determine pseudoranges to various GNSS sources and compute the position of the receiver using the calculated pseudoranges, timing and ephemeris data. The pseudoranges are time delay (a.k.a. code phase) values measured between the received signal from each GNSS source and a local reference function. Acquisition of the GNSS source signal can take up to several minutes and must be accomplished using a sufficiently strong received signal to achieve low error rates.
Each GNSS source transmits a RF signal which is modulated by a code whose phase is used to provide the time (and thus distance) accuracy needed for positioning applications. In some systems, a pseudorandom noise (PN) ranging code is unique to each GNSS source (e.g., the U.S. GPS system), while in others a common code is used while the carrier frequency is unique to the GNSS source (e.g., the Russian Glonass system). In addition, the RF signal is also modulated by a navigation data message. In one example, GPS sources transmit using the same carrier frequency at the same time. Thus, the GNSS receiver discriminates each GNSS source signal by the well-known code division multiple access (CDMA) technique. The PN codes are selected to be nearly orthogonal (uncorrelated) to each other. For purposes of description and not limitation, the following description concentrates primarily on CDMA satellite systems.
In order to determine position information for a GNSS receiver, the time between the transmission of the signal and its reception at the receiver is needed. The time difference may be determined partially by (e.g.) demodulating the data stream of the navigation message and using frame and bit synchronization, while the PN code phase offset of the received signal is used to determine time more precisely. The initial determination of the code phase offset for a particular satellite is referred to as acquisition of that satellite.
In general, to acquire and track a desired GNSS source that is in common view with several other GNSS sources, a GNSS receiver can replicate the unique PN code and the carrier signal, including Doppler offsets, to generate a two-dimensional reference function. The GNSS receiver can then correlate the received signal (which is a composite of many GNSS source signals received by the GNSS receiver) with the two-dimensional reference function to yield a two-dimensional correlation function indexed in time delay and Doppler offset. Maximum correlation occurs when the time delay and Doppler offset of the reference function matches the time delay of the incoming PN code and the Doppler offset of the desired signal (extracted from the received signal). After acquisition, the GNSS receiver maintains track of the desired signal by continuous adjustment of the time delay and Doppler offset (which varies with the relative velocity between the receiver and the satellite).
In general, signals transmitted from different GNSS sources do not significantly interfere with each other since they use unique PN codes (which are nearly orthogonal to one another) and/or unique carrier frequencies. The interference level depends on the relative amplitude of the received signal. Under some conditions, one or more signals transmitted by the GNSS sources can be attenuated relative to signals transmitted by other GNSS sources. The presence of strong GNSS signals produces interference that can reduce the ability to track weaker GNSS signals.
In one example, cross-correlation spurs (spurious maxima when correlating one PN code with another PN code that may be wrongly declared as true auto-correlation peaks and may thus cause false acquisition) are generated when an interfering GNSS source is received at certain frequencies relative to the search frequency of the desired GNSS source. In one example, since the C/A (coarse/acquisition) PN codes of the GPS satellites have a period of 1 ms, the most significant cross-correlation maxima occur when the Doppler difference between the interfering GPS satellite and the desired source is a multiple of 1 kHz (the reciprocal of the PN code period). Additionally, there may be weaker cross-correlation spurs at other frequencies. These cross-correlation spurs can cause false acquisitions, for example under certain Doppler difference and/or antenna gain conditions. In one example, a desired source may be at a low elevation angle, may suffer multipath loss, have greater atmospheric loss and/or be received with lower antenna gain all compared to an interfering source at a high elevation angle. In this example, the relative cross-correlation maxima between the desired reference signal and the undesired signal (from the interfering source) may be relatively high which causes a false acquisition (that is, the receiver may determine that it has acquired satellite A, when instead the received signal that produces the spurious peak in the correlation function was from satellite B). As a consequence of these spurious cross-correlation maxima, the GNSS receiver may falsely acquire the undesired signal which cannot subsequently be tracked, generally leading to an incorrect position fix.